1. Field of the Invention
The present invention relates to a fixing unit controlling apparatus.
2. Description of the Related Art
Electrophotographic printers are provided with a fixing unit for fixing a toner image which has been transferred to a print medium. The fixing unit includes a heating roller having a built-in heater, a pressure roller opposing the heat roller, and an oil roller. The oil roller is impregnated with an offset-preventing liquid such as dimethyl silicone and is in contact with the heat roller.
The fixing unit is controlled so that the surface temperature of the heat roller is maintained constant.
FIG. 40 illustrates changes in the surface temperatures of the heat roller and pressure roller when a continuous printing is performed.
FIG. 41 illustrates changes in the surface temperatures of the heat roller and pressure roller when a plurality of pages are printed intermittently with long intervals between pages. FIGS. 40 and 41 plot time as the abscissa and temperature as the ordinate. Curves labeled "Th" indicate the surface temperature of the heat roller and curves labeled "Tp" represent the surface temperature of the pressure roller.
Referring to FIG. 40, the surface temperature Th of the heat roller is maintained substantially the same at all times if a printing is performed continuously on a plurality of pages of print medium after the fixing unit has become ready for a fixing operation. However, some of the pressure roller heat is lost to each print medium page. Therefore, the pressure roller surface temperature slowly decreases. In contrast, if the printing is performed intermittently with a sufficiently long period of time between adjacent pages, an excess heat is transferred from the heat roller to the pressure roller, so that the surface temperature Tp of the pressure roller will become too high.
Referring to FIG. 41, once the fixing unit has become ready for a fixing operation, the surface temperature Th of the heat roller is maintained substantially the same at all times if a printing is performed intermittently where the fixing unit is stopped every time a page is printed. However, the surface temperature of the pressure roller will increase with time at a slow rate. If the printing is performed with much longer intervals between pages, the surface temperature Th of the heat roller is maintained substantially constant, while the surface temperature of the pressure roller will still increase at a slow rate.
Changes in the surface temperature Th of the pressure roller is a critical factor in color printing. A change of about 10 degrees in the surface temperature Th not only causes the gloss of a color image which deteriorates the quality of the color image, but also leads to "offset phenomenon".
One way of addressing this problem may be to provide a heater in the pressure roller just as in the heat roller so as to maintain the surface temperature Tp within a certain range. This approach requires two heaters which add to the manufacturing cost of the electrophotographic printer. The additional heater consumes an additional electric power, thereby increasing the running cost.